1. Field of the Invention
This invention relates to an image information read-out apparatus, and more particularly to a shading correction which is carried out by an image information read-out apparatus on an image read out by the apparatus.
2. Description of the Related Art
There have been various known image information read-out apparatuses in which a light beam is caused to scan a recording medium on which an image is recorded and image information representing the image is read out by detecting the reflected light, transmitted light or light emitted from the recording medium. For such image information read-out apparatuses, as well as for image scanners, facsimile machines or the like, a radiation image information read-out apparatus may be used in a radiation image recording and reproducing system using a stimulable phosphor sheet.
In such image information read-out apparatuses, fluctuation in the image signal in the main scanning direction (so-called shading) can occur due to nonuniformity in the projecting angle of the light beam and/or nonuniformity in the sensitivity of the photodetector. In order to overcome this problem, various propositions have been made. Though we have proposed a method in which the sensitivity of the photodetector is corrected to avoid influence of shading, shading is recently mainly compensated for by adding or subtracting correction data to or from a digitized image signal (digital shading correction). See, for instance, Japanese Unexamined Patent Publication No. 9(1997)-69940.
However when a shading correction is carried out in a digital region, the following problem arises. That is, for example, when the shading characteristics are as shown in FIG. 4, the effective dynamic range is narrower than the natural dynamic range by an amount corresponding to the shading (i.e., the difference between the maximum value and the minimum value of the shading data).